X-ray tomographic, three-dimensional (3D) volumetric image data set acquisition apparatuses, and methods to translate these data into voxel matrices of X-ray attenuation values are known. The results are generally presented as various two-dimensional (2D) slice images of relative contrast of X-ray attenuation to provide geometrical and structural information about the object under investigation. Typically, the diagnostic or treatment targeting or other disease management activities only make use of the geometry and structure contrast of slice images. The shapes of the matrix voxels are generally asymmetric with relatively small dimensions (high spatial resolution) in the (x, y) plane perpendicular to the axis of rotation, and a larger dimension (lower spatial resolution) in the z-direction parallel to the axis of rotation. The ratio of the largest-to-smallest voxel dimension often exceeds a factor of four. The result is voxels that look more like rectangular soda straws than cubes. In digital tomosynthesis, this elongation of the z-dimension is even more significant due to the limited geometry directions from which the measured data are acquired.
It is known that X-ray attenuation is primarily a result of three types of interactions, namely photoelectric absorption, Compton scattering, and pair production. All the interactions have dependencies on the incident X-ray photon energy, object atomic number, and object mass density. Thus tomography determined X-ray attenuation values for voxels are an integral, complex weighted “average” over the atomic numbers, mass densities, and the energy distribution of the incident X-rays. The energy distribution of an incident X-ray changes with the distance the X-ray beam has to penetrate the object to reach the voxel being measured. In computed tomography, the resulting voxel attenuation values are typically calibrated using the average X-ray attenuation of water as the zero reference and free space attenuation as a value of minus 1000, expressed in Hounsfield units (HU).
Most recently, cone beam computed tomography is used to provide volumetric HU data sets with voxels of more symmetric dimensions in all three directions. The largest dimension can be in the range of 0.3 mm or larger, and the asymmetry ratio values can be less than two. Micro computed tomography systems have been made with reconstructed voxel dimensions of 100 μm or less on the longest side, but only in configurations for imaging small animals such as mice, rats, or rabbits.
The valuable properties of the distributions of attenuation HU values of voxels in specified regions of interests in mammals have not been used in cancer detection, diagnosis, staging, treatment planning, delivery, and monitoring. Further developments are needed to acquire data that can be reconstructed into small voxels approximately in cubic shapes with a system that can be used for human patients.